Preheat oven to 350 degrees F. In a large bowl,cream butter and sugar with an electric mixer. Then beat in eggs and vanilla. In a separate bowl,combine flour,pecans,cocoa, baking powder and salt.
Add to butter mixture,blending thoroughly. Spread mixture into a greased 9- x 13-inch baking pan. Bake 15 to 20 minutes or until a wooden toothpick inserted in center comes out clean. Sprinkle marshmallows evenly over top and bake an additional 3 minutes.
Remove from oven to cool. In a small saucepan, melt chocolate chips and peanut butter together over low heat. Remove from heat and stir in cereal. Spread mixture over cooled uncut bars. Chill,then cut into 3 x 6 inch bars. Keep refrigerated.